1. Field of the Invention
The present invention relates to a heat mode type optical information recording medium which has an organic dye thin film and wherein recording and readout are carried out by causing a change of state with a high density energy beam.
2. Discussion of Related Art
Heretofore, information recording media wherein recording and readout of information are carried out by irradiating a rotating disc-shaped information recording medium with a laser beam have been known. As recording layers in these information media, there are, for example, those recording layers proposed wherein a metal having a low melting point and a dielectric substance are used. However, these recording layers have disadvantages such as poor preservability, low separating ability, low recording density and high manufacturing cost. Recently, it has been proposed and practised that dye thin films whose physical properties can be changed with light of relatively long wavelength are used in the recording layers. However, dyes which have absorption bands in long wavelength generally have problems, for example a problem that they have only low stabilities against heat and light. Thus, it is the present state of things that recording layers having recording characteristics which are stable and satisfactory over a long period have not yet been developed.
Indolenine series heptamethinecyanine dyes as disclosed, for example, in Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") No. 59-202892 have been known as dyes having the best recording characteristics and stability among dyes having hitherto been disclosed for recording layers.
Further, U.S. Pat. No. 4,460,665 discloses an optical information recording medium wherein a cyanine dye is used, and J.P. KOKAI Nos. 59-24692, 59-85791 and 59-67092 disclose optical information recording media wherein indolenin cyanine dyes are used. In this connection, it has been known that the stability of recording layers comprising a cyanine dye is greatly influenced by the structure of its counter anion (for example, compare Comparative example 1 with Comparative example 4 of J.P. KOKAI No. 59-202892), and most stable recording layers have been obtained when the perchlorate ion had been used among the counter anions as previously disclosed. However, most perchlorate are generally explosive and moreover DSC (Differential Scanning Calorimetry) measurement has shown that perchlorates of cyanine dyes decompose with very large thermal emission, and thus it has been suggested that they are in high danger of explosion during handling (e.g., preparation). Therefore, optical information recording media have been desired comprising a cyanine dye which have a recording performance and stability of the recording layer equal to or superior to that of perchlorates of cyanine dyes, and are further free from danger of explosion.